Unmanned Systems Technology 026 I Tecdron TC800-FF I Propellers I USVs I AUVSI 2019 part 1 I Robby Moto UAVE I Singular Aircraft FlyOx I Teledyne SeaRaptor I Simulation & Testing I Ocean Business 2019 report

Propellers | Focus Some manufacturers and users might X-ray props to identify internal damage, but generally a propeller is replaced as soon as signs of wear or fatigue are suspected. Design innovations The past few years have seen new solutions and approaches to blade configuration emerge, particularly with regard to reducing propeller noise and vibration. For example, while winglets on the tips of propeller blades were being hypothesised as feasible only two years ago, these days they are a common sight on multi-copters. A wingletted blade will induce less drag than a ‘flat’ blade of equivalent material volume, and generate less noise, as the shorter distance between the blade hub and its tip means the effective tip speed is kept further from the speed of sound without sacrificing lift. This shorter hub-tip distance also decreases the risk of injury from accidental blade contact. However, as this technology is still fairly new, real- world data for modelling and optimising the exact performance characteristics of wingletted propellers might be hard to come by, compared with conventional planform props. Also surging in popularity is the use of hub-less, rim-driven propellers for UUVs. Here, the outer sections of the propeller blades are mounted to a ring that is directly driven by a motor in the rim. This typically means the ring is integrated inside a permanent magnet rotor, with the motor stator encircling it. With the tips of these hub-less thrusters pointing into the centre of rotation, tip speed – and therefore ‘noise’ and vibration – are greatly reduced compared with typical hub-driven propellers for AUVs and ROVs. These rim-driven thrusters are also simpler, more reliable and more energy-efficient, as they eliminate the need for a shaft or gearbox. Such mechanical simplicity also means the thrusters can be more easily configured for azimuthing. As a result, even USV designers and autonomous shipping developers are becoming more interested in these thruster-propeller systems, for the 360° directional control and station-keeping abilities they can provide for complex surveys and manoeuvres, compared with conventional thrusters limited to pointing off the stern of their vessels’ hulls. Their design was a rare offering a few years ago, often being additively printed to capture their then-unusual geometry. As demand has risen though, justifying the case for bulk production, they are now widely available in a range of sizes and power ratings, and in metals and injection-moulded plastics. We might see further innovations for aerial props in the future. For example, studies have been conducted in the past two years into UAV propellers with